THIS INVENTION relates to catalysts. It relates in particular to a process for preparing a catalyst precursor.
According to a first aspect of the invention, there is provided a process for preparing a catalyst precursor, which process comprises
subjecting, in an initial treatment stage, a slurry comprising a porous oxidic catalyst support or carrier, an active catalyst component or its percursor, and water, to treatment at elevated temperature and at sub-atmospheric pressure such that impregnation of the support or carrier with the active catalyst component or its precursor and partial drying of the impregnated support or carrier occurs, with the initial treatment stage not continuing beyond a point where the impregnated carrier or support has a loss on ignition (LOI) which is less than 1.2 times its loss on ignition at incipient wetness (LOIiw);
thereafter, in a subsequent treatment stage, subjecting the partially dried impregnated support or carrier to treatment at elevated temperature and at sub-atmospheric pressure such that the temperature in the subsequent treatment stage exceeds that in the initial treatment stage and/or the sub-atmospheric pressure in the subsequent treatment stage is lower than that in the initial treatment stage, thereby to obtain more vigorous drying of the impregnated support or carrier in the subsequent treatment stage than in the initial treatment stage, with a dried impregnated carrier or support thereby being produced; and
calcining the dried impregnated carrier or support, to obtain the catalyst precursor.
The resultant catalyst precursor is, in practice, subjected to reduction, in order to obtain a catalyst.
The porous oxidic catalyst support may, in particular form. In principle, any commercially available oxidic catalyst support can be used. Examples of catalyst supports that can be used are alumina (Al2O3) and titania (TiO2). The support preferably has an average pore diameter between 8 and 50 nanometers, more preferably between 10 and 15 nanometers. The support pore volume may be between 0.1 and 1 ml/g, preferably between 0.3 and 0.9 ml/g. The average particle size may be between 1 and 500 micrometers, preferably between 10 and 250 micrometers, still more preferably between 45 and 200 micrometers. Alumina is preferred as the support, and the invention is described further hereunder with reference to alumina as the suport.
While the active catalyst component can, at least in principle, be any known Fischer-Tropsch active component such as cobalt (Co), iron (Fe), nickel (Ni) or ruthenium (Ru); however, cobalt (Co) is preferred. In particular, a cobalt precursor can be used. Still more particularly, cobalt nitrate (Co(NO3)2xc2x76H2O) is preferably used.
From 1.18xy to 1.82xy kg Co(NO3)2xc2x76H2O may initially be used in the initial treatment stage, where x is the BET pore volume of the alumina support in ml/g, and y is the mass of alumina support to be impregnated, in kg.
The process may include initially dissolving the Co(NO3)2xc2x76H2O in the water, which is preferably distilled water. Sufficient water may be used such that the volume of the solution is greater than xyl, and preferably is about 2xyl.
In one version of the invention, this solution may be heated to a temperature between 60xc2x0 C. and 95xc2x0 C., with the support then being added to the solution at atmospheric pressure, to form the slurry. The slurry may be mixed, preferably on a continuous basis, eg by means of an internal rotating screw in a conical vacuum drier in which the slurry is held.
In the initial treatment stage, vacuum may then gradually be applied to the slurry, preferably under continuous mixing, eg stirring, thereof, at a temperature between 60xc2x0 C. and 95xc2x0 C., which may be the same as the temperature to which the solution is initially heated, or different therefrom. This constitutes the initial treatment of the slurry, and it is important that the initial treatment be effected in a gradual manner, ie excessive boiling of the slurry is to be avoided.
The sub-atmospheric pressure or vacuum that is applied during the initial treatment stage may be down to 20 kPa(a), ie between atmospheric pressure and 20 kPa(a). Typically, the vacuum may be about 20 kPa(a) for a slurry temperature of 60xc2x0 C., and about 83 kPa(a) for a slurry temperature of 95xc2x0 C.
The initial treatment stage is preferably continued until the loss on ignition (LOI) of the impregnated alumina support is 1.2 times LOIiw, ie 1.2 times the LOI at the point of incipient wetness (iw). Incipient wetness occurs when all the pores of the support are filled with liquid and there is no excess moisture, over and above the liquid required to fill the pores, present. Typically, the initial treatment time will be up to 3 hours or more.
Loss on ignition (LOI) is defined as the mass % loss observed during complete calcination, ie during decomposition to Co3O4/Al2O3, experimentally to be determined as the mass % loss observed during calcination at 400xc2x0 C., ie at a temperature sufficiently high to ensure quantitative decomposition of cobalt nitrate to Co3O4, but too low in order to effect the undesired formation of cobalt aluminates.
The LOI value at the state of incipient wetness, ie LOIiw, can be expressed as a function of the pore volume of the support as well as the amount of catalyst active component to be impregnated. The pore volume of the support, prior to impregnation, is as stated hereinbefore, equal to x ml/g. The amount of Co(NO3)2xc2x76H2O to be impregnated is M gram per gram of support material, and will fall within the range: 1.18 x to 1.82 x gram per gram support material. M is thus determined by the amount of Co(NO3)2xc2x76H2O initially used. The LOI value at the state of incipient wetness can be calculated as follows:
LOIiw=100 ((0.20M +x)/(0.475M+x+1))xe2x80x83xe2x80x83(1) 
This shows that the LOI at the state of incipient wetness is dependent on the pore volume of the support and the amount of Co(NO3)2xc2x76H2O used for the catalyst preparation.
The gradual drying procedure until the LOI is 1.2 times LOIiw ensures that about 83% of the cobalt nitrate is quantitatively drawn into the pores of the alumina support without the occurrence of localized saturation, which results in premature crystallization of cobalt nitrate.
At a moisture point somewhat above incipient wetness, ie when LOI of the impregnated alumina support is 1.2 times LOIiw, aggressive evacuation, eg increased vacuum pump suction capacity when a vacuum pump is used, may be applied, in the subsequent treatment stage; at the same time, it is ensured that the support temperature is controlled at between 60xc2x0 C. and 95xc2x0 C. Thus, when a vacuum drier, in which the impregnated support is contained in the of a bed, is used, an increased setting of the vacuum drier wall temperature is used, thereby ensuring that the bed temperature is controlled between 60xc2x0 C. and 95xc2x0 C., under continuous mixing, eg stirring. This constitutes the subsequent treatment in which more forceful drying of the impregnated support takes place. Once the point where LOI=1.2 times LOIiw has been reached, the more forceful vacuum drying during the subsequent treatment stage preferably proceeds in an uninterrupted fashion, preferably at the conditions:
 greater than 60xc2x0 C., but not higher than 95xc2x0 C., and at the minimum pressure which is attainable, with this pressure being  less than 20 kPa(a)
Vacuum drying under these specific conditions should be maintained until a clearly defined maximum required LOI value is reached, which value depends on the need to store the dried material for a certain period of time before calcination can be executed, as hereinafter described, and this maximum required LOI value is smaller than, or equal to, 0.90 times LOIiw.
The calcination of this dried impregnated support may be effected in a fluidized bed, or a rotary kiln, calciner at a temperature from 200xc2x0 C. to 300xc2x0 C., preferably at about 250xc2x0 C.
The process thus involves using a slurry, ie an excess of moisture, to achieve impregnation of the support; thereafter drying the impregnated support in a gradual manner during the initial treatment stage until 1.2 times LOIiw; whereafter the more forceful drying of the subsequent treatment stage is effected until the maximum required LOI value is attained.
Sufficient cobalt nitrate may initially be used to obtain a cobalt loading between 5 g Co/100 g support and 70 g Co/100 g support, preferably between 20 g Co/100 g support and 40 g Co/100 g support, more preferably between 25 g Co/100 g support and 35 g Co/100 g support.
The maximum cobalt loading attainable in a single support impregnation step as hereinbefore described is as given in Table 1:
The optimum cobalt loading is defined as the maximum cobalt loading at which the cobalt utilization is still optimum. In the case of the Fischer-Tropsch application of a Co/Al2O3 catalyst, it was determined that a direct proportionality between cobalt loading and catalyst productivity existed up to a cobalt loading of 30 g Co/100 g Al2O3, for a Al2O3 support material with a pore volume of about 0.5 ml/g, and an average pore diameter of 12 nanometer.
It is clear from Table 1 that an optimum cobalt loading of 30 g Co/100 g Al2O3 cannot be achieved on a Al2O3 support material with a pore volume of 0.5 ml/g, in a single impregnation step. In order to achieve a cobalt loading of 30 g Co/100 g Al2O3 in a single impregnation step, an Al2O3 support material with a minimum pore volume of 0.84 ml/g is required. In accordance with the invention, the calcined catalyst precursor obtained from the abovementioned initial or first impregnation step (ie 18.4 g Co/100 g Al2O3 in the case of a support material with a pore volume of 0.5 ml/g), must be subjected to a further impregnation, drying and calcination in a second impregnation step. The second impregnation step may then comprise
subjecting, in an initial treatment stage, a slurry comprising the calcined material of the first impregnation step, an active catalyst component or its precursor, and water, to treatment at elevated temperature and at sub-atmospheric pressure such that impregnation of the calcined material with the active catalyst component or its precursor and partial drying of the impregnated material occurs, with the initial treatment stage not continuing beyond a point where the impregnated material has a LOI which is less than 1.2 times its LOIiw;
thereafter, in a subsequent treatment stage, subjecting the partially dried impregnated material to treatment at elevated temperature and at sub-atmospheric pressure such that the temperature in the subsequent treatment stage exceeds that in the initial treatment stage and/or the sub-atmospheric pressure in the subsequent treatment stage is lower than that in the initial treatment stage, thereby to obtain more vigorous drying of the impregnated material in the subsequent treatment stage than in the initial treatment stage, with a dried impregnated material thereby being produced; and
calcining the dried impregnated material, to obtain the catalyst precursor.
As also stated hereinbefore, the catalyst precursor is, in practice, reduced, to obtain a catalyst.
When a Co(NO3)2xc2x76H2O precursor is used in the first impregnation step, then the same precursor is preferably used in the second impregnation step. The amount of Co(NO3)2xc2x76H2O used during the second impregnation step may be from 1.18x1y1 to 1.82x1y1 kg where x1 is the BET pore volume of the calcined material from the first impregnation step, in ml/g, and y1 is the mass of calcined material from the first impregnation step to be impregnated in the second impregnation step, in kg. This range of cobalt nitrate allows for a limited flexibility with respect to the cobalt loading of the resultant catalyst to be broadened by support tailoring. For example, when alumina is initially used as the support material, Table 2 provides the correlation between the pore volume of the starting alumina, ie x ml/g, and the empirically derived maximum attainable cobalt loading in a two-step impregnation procedure as hereinbefore described.
For example, if the objective is a final catalyst having a cobalt loading of 30 g Co/100 g Al2O3, the starting alumina support must have a pore volume xe2x89xa70.43 ml/g.
This amount of cobalt nitrate may initially be dissolved in water, which is preferably distilled water. Sufficient water may be used such that the volume of the solution is  greater than x1y1l, preferably about 2x1y1l. This solution may then be heated to a temperature between 60 and 95xc2x0 C. To this solution, the final inventory of y1 kg of the first impregnation step material, ie the catalyst precursor of the first impregnation and calcination step, may be added at atmospheric pressure, whilst continuous mixing of the slurry is maintained, eg by means of an internal rotating screw in a conical vacuum drier.
In the initial treatment stage of the second impregnation step, vacuum may then gradually be applied to the slurry, preferably under continuous mixing, eg stirring, thereof, at a temperature between 60xc2x0 C. and 95xc2x0 C., which may be the same as the temperature to which the solution is initially heated, or different therefrom. This constitutes the initial treatment stage of the slurry, and it is important that the initial treatment be effected in a gradual manner, ie excessive boiling of the slurry is to be avoided.
The initial treatment stage of the second impregnation step is preferably continued until the LOI of the impregnated material is reduced to a point where it is 1.2 times LOIiw. Typically, the initial treatment time will be up to 3 hours or more.
The sub-atmospheric pressure or vacuum that is applied during the initial treatment stage may be down to 20 kPa(a), ie between atmospheric pressure and 20 kPa(a). Typically, the vacuum may be about 20 kPa(a) for a slurry temperature of 60xc2x0 C. and about 83 kPa(a) for a slurry temperature of 95xc2x0 C.
As stated hereinbefore, this gradual drying procedure until the LOI is 1.2 times LOIiw ensures that about 83% of the cobalt nitrate is quantitatively drawn into the pores of the calcined material without the occurrence of localized saturation, which results in premature crystallization of cobalt nitrate. At a moisture point somewhat above incipient wetness, ie at the point where LOI is 1.2 times LOIiw, aggressive evacuation, eg increased vacuum pump suction capacity when a vacuum pump is used, may be applied, in the subsequent treatment stage of the second impregnation step; at the same time, it is ensured that the support temperature is controlled at between 60xc2x0 C. and 95xc2x0 C. Thus, when a vacuum drier, in which the impregnated material is contained in the form of a bed, is used, an increased setting of the vacuum drier wall temperature is used, thereby ensuring that the bed temperature is controlled between 60xc2x0 C. and 95xc2x0 C., under continuous mixing, eg stirring. Preferably, maximum vacuum ( less than 20 kPa(a)) is applied, whilst simultaneously ensuring that the bed temperature does not drop below 60xc2x0 C., under continuous mixing. This constitutes the subsequent treatment stage. Once the point where LOI=1.2 times LOIiw has been reached, vacuum drying during the subsequent treatment stage preferably proceeds in an uninterrupted fashion, preferably at the conditions:
 greater than 60xc2x0 C., but not higher than 95xc2x0 C., and at the minimum pressure which is attainable, with this pressure being  less than 20 kPa(a).
Vacuum drying under these specific conditions should be maintained until a clearly defined maximum LOI value is reached, which value depends on the need to store the dried material for a certain period of time before calcination can be executed, as hereinafter described, and this maximum required LOI value is smaller than, or equal to, 0.90 times LOIiw.
The calcination of this dried impregnated material may be effected in a fluidized bed, or a rotary kiln, calciner at a temperature from 200xc2x0 C. to 300xc2x0 C., preferably at about 250xc2x0 C.
During the first treatment stage of the first impregnation step and/or during the first treatment stage of the second impregnation step, a water soluble precursor salt of palladium (Pd) or platinum (Pt) may be added, as a dopant capable of enhancing the reducibility of the active component. The mass proportion of the palladium or platinum metal to the cobalt metal may be between 0.01:100 to 0.3:100.
It has hitherto generally be known to those skilled in the art that high drying rates during catalyst support impregnation and drying will result in catalysts with a homogeneous macroscopic distribution of the active component in the catalyst particles, ie an absence of an eggshell distribution.
Surprisingly, it has now been found that even if the macroscopic distribution of the active component is very homogeneous, controlling the drying rate of the slurry to a specified drying profile from the point of 1.2 times LOIiw during the first and second impregnation steps, a catalyst with a more desired activity, is consistently obtained. The slope of the drying profile, ie the drying rate, at the point of incipient wetness should preferably be greater than (0.048 hxe2x88x921) LOIiw. The slope of the drying profile is determined at the point of incipient wetness. This may be done by matching the experimental data to an empirical equation, eg y=a 1nx+b, and calculating the derivative at the point of incipient wetness. After having determined a suitable equation to fit the experimental data, this type of equation should be used exclusively to calculate the drying rate, ie the tangent at the point of incipient wetness, for all drying profiles.
The impregnation and drying of the catalyst support in the sub-atmospheric environment, ie the initial and subsequent treatment stages of the first and second impregnation steps, can be performed in, for example, a conical vacuum drier with rotating screw or a tumbling vacuum drier, preferably a conical vacuum drier. The desired drying profile can be achieved by decreasing the sub-atmospheric pressure, by more efficient mixing, by increasing the temperature of the vacuum drier wall, or by the introduction of hot air during the subsequent treatment stage, but preferably is achieved by more efficient mixing.
It has also hitherto generally been known to those skilled in the art that the impregnated and dried material need not necessarily be calcined immediately after impregnation and drying thereof. A less desired catalyst activity has, however, been observed if storage occurred between the catalyst support impregnation/drying and product calcination.
Surprisingly, it has now been found that if the drying profile in accordance with the invention is met during the subsequent treatment stages, and drying is immediately continued under the sub-atmospheric pressure at temperatures between 60xc2x0 C. and 95xc2x0 C. to LOI values lower than 0.9 LOIiw, the maximum allowable storage time at ambient conditions in a dry environment between the catalyst support impregnation/drying and the catalyst precursor calcination is a direct function of LOIunload, ie the LOI at which the impregnated support drying, ie the subsequent treatment stage, was trminated and the dried impregnated material unloaded from the vacuum drying equipment. The maximum allowable storage time before calcination should preferably be less than ((xe2x88x928.1/LOIiw)LOIunload+26.2) hours, which thus results in a catalyst that has a more desired activity.
Instead of, in the first and/or the second step, heating the solution of cobalt nitrate in water to the temperature between 60xc2x0 C. and 95xc2x0 C. and then adding the particulate support thereto, the support may be added to the solution at ambient temperature, whereafter the temperature of the slurry is increased to a minimum of 60xc2x0 C. and a maximum of 95xc2x0 C. prior to evacuation to a vacuum of xe2x89xa720 kPa(a). During the initial treatment, the temperature may then be increased slowly, to ensure that the gradual treatment, ie without excessive boiling of the slurry, is effected. Once the stage described by LOI=1.2 times LOIiw has been reached, more vigorous treatment is effected by aiming for a slurry temperature xe2x89xa760xc2x0 C., preferably 95xc2x0 C., whilst applying maximum allowable suction capacity affordable by the vacuum pump, effecting a drying rate in excess of (0.048 hxe2x88x921) LOIiw.
The catalyst obtained is particularly suitable for use as a Fischer-Tropsch catalyst to catalyze the Fischer-Tropsch reaction of a synthesis gas, comprising hydrogen and carbon monoxide, to produce Fischer-Tropsch products.